The advantages of all optical signal processing for high bit rate systems are being increasingly recognised, primarily for their potential for performing switching and logic operations, for example, at extremely rapid rates since optical frequencies are of the order of 10.sup.14 Hz. In order to exploit this potential it is necessary to construct devices whose response is determined by the optical input. This essentially means a device which responds differently to different intensities of input optical signal, that is, an optically non-linear device.
One approach to obtaining such a non-linear device is to employ an interferometer arrangement comprising an optical splitter which splits an input signal into two portions of different intensities which travel along an optically non-linear transmission medium, or media, to an optical combiner. An example of such device is described in the applicant's co-pending application EP 0265233 Al which comprises an optical waveguide formed from an optical fibre having non-linear refractive index coupled to a first pair of ports of an optical coupler. In this configuration the coupler acts as both the splitter to split an input optical signal into two counter-propagating signal portions of different intensity round a common, non-linear medium and as the combiner.
The output from the combiner is dependent on the final relative phase shift of the two portions after they have propagated from the splitter to the combiner. In particular, as discussed in detail in the above referenced application, the output from the ports of the coupler is an oscillatory, periodic, function of the intensity of the input signal. The difference of the maximum and minimum intensities between which the output signal oscillates, the contrast ratio, is a function of the asymmetry of the splitter. The same oscillatory behaviour can also be realised in other interformeter arrangements, for example a Mach-Zehnder interferometer. Such interferometers, which include a non-linear transmission medium, will be referred to hereinafter as non-linear interferometers.
Such known non-linear interferometers can be used for switching and logic applications but they suffer from the disadvantage that intensity of the input signal must be controlled within predetermined bounds to ensure that the intensity of the output signal remains close to some predetermined threshold level which determines that the output is of a given logic level. If the intensity of the input signal increases too much the output of the device reverts to the other logic level.